Optical compensators are used to improve the optical properties of liquid crystal displays (LCD), such as the contrast ratio and the grey scale representation at large viewing angles. For example in uncompensated displays of the TN or STN type at large viewing angles often a change of the grey levels and even grey scale inversion, as well as a loss of contrast and undesired changes of the color gamut are observed.
An overview of the LCD technology and the principles and methods of optical compensation of LCDs is given in U.S. Pat. No. 5,619,352, of which the entire disclosure is incorporated into this application by way of reference.
As described in U.S. Pat. No. 5,619,352, the contrast of a display at wide viewing angles can be improved by a negatively birefringent C-plate compensator. However, such a compensator does not improve the greyscale representation of the display. On the other hand, U.S. Pat. No. 5,619,352 suggests using a birefringent O-plate compensator to suppress or even eliminate grey scale inversion and improve the grey scale stability.
The terms ‘O-plate’ and ‘A-plate’ as used in U.S. Pat. No. 5,619,352 and throughout this invention have the following meanings. An ‘O-plate’ is an optical retarder utilizing a layer of a positively birefringent (e.g. liquid crystal) material with its principal optical axis oriented at an oblique angle with respect to the plane of the layer.
An ‘A-plate’ is an optical retarder utilizing a layer of uniaxially birefringent material with its extraordinary axis oriented parallel to the plane of the layer, and its ordinary axis (also called ‘a-axis’) oriented perpendicular to the plane of the layer, i.e. parallel to the direction of normally incident light.
As an O-plate retarder for example an optical retardation film comprising a layer of a liquid crystal or mesogenic material with tilted structure can be used. As an A-plate retarder an optical retardation film may comprise a layer of a positively birefringent liquid crystal or mesogenic material with planar orientation.
Those retardation films are commonly used in liquid crystal displays to convert between linear and circular polarized light. The skilled artisan is aware that reactive mesogen layers can be created to provide such retardation layers. For example, RMS03001 (from Merck KGaA, Darmstadt, Germany) as a commercially available solution of reactive mesogens can be spin coated and photo-polymerised to provide a planar aligned nematic film. By varying the coating conditions, it is possible to produce films of different thickness and so produce half-wave and quarter wave retardation films.
Usually, reactive mesogen layers require an alignment layer or rubbed plastic substrate to align in the planar state. In this regard, two main methods are currently used in the display industry to align liquid crystals for optical film applications:
(i) Rubbing process, wherein a plastic substrate or alignment layer is rubbed in one direction providing alignment direction for coated liquid crystals. The alignment quality varies depending on the rubbing process and the properties of the substrate or film. The rubbing process is difficult to optimize and can produce variable results. Furthermore, the rubbing process is considered an unfavorable process by LCD producers because it can produce particles that are difficult to control in high-class clean rooms.(ii) Photoalignment process, as described in U.S. Pat. No. 7,364,671 B2, wherein a dichroic photoinitiator is photo-oriented while maintaining conditions that the polymerisation or cross-linking of the polymerisable liquid-crystalline material is essentially inhibited. The photoalignment and polymerisation step are carried out in two different steps and under different conditions. Accordingly, such photoaligning layers can be difficult to prepare due to the requirements that the production conditions have to be adjusted concerning the individual composition of various liquid-crystalline materials. In addition, an annealing step is usually required to allow liquid crystals to align fully. Consequently, photoaligning layers following this process are expensive.
Therefore, there is still the need for alternative production methods, which do not have the drawbacks of prior art methods or have them to a less extent.
One aim of the present invention is to provide a one-step method of production for optical compensators, which
a) is in particular suitable for mass production,
b) is applicable for a broad range of polymerisable liquid-crystalline materials,
c) does not require an alignment layer like e.g. rubbed polyimide layer,
d) allows patterning of polymer films,
e) allows overcoating a chosen layer without the need for an extra alignment layer, and
f) allows producing thick films with a uniform alignment.
Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
Surprisingly, the inventors have found that the above-described problems can be solved by the present invention, which removes the need for an alignment layer or rubbing process to provide liquid crystal alignment for planar aligned optical films, and which provides a way of producing planar or tilted aligned film without the need for an alignment layer and/or rubbing.
The polymer films can be created from reactive mesogens coated in the isotropic phase. A polarisation state sensitive photoinitiator is required in combination with UV polarised light and heat to induce planar or tilted alignment in the resultant optical film. The process of making such a film can be completed in one step using heat and polarised UV light. As such, they can be coated on many different substrates (e.g. plain glass, colour filters, plastic substrates) without further treatment, producing e.g. planar A-type retarder films or tilted O-type retarder films.